Guys, Thanks for all the responses both on-line here in the group and off-line. Several people pointed me to Jim Brown's (K9YC) paper, including just a few minutes ago, Jim Brown himself (thanks).
I grabbed the paper last night but was too tired from a half-bottle of champagne to do anything but briefly scan the parts about the common mode chokes. Also, I found a power-point talk by K9YC that I will also go through this morning. So, I am pretty sure I will end up doing the Jim Brown solution. That would be an RG8X feed using one of his common mode toroid chokes (though, I do need to read the paper in detail). I guess finding a source for the ferrite toroids is next on my list although I think I saw one referenced in one of the responses (digikey). I suppose best placement for the choke is at the center feed point of the dipole but I am a little concerned about the weight. Maybe this is not an issue. By the way, I make my own center insulators with nice screw on brackets and SO-239 connectors. The plastic I use is very tough and I buy it from Tap Plastics. PEH On Jan 1, 2012, at 9:13 AM, Phil Hystad wrote: > Don, > > goals: > > 1. Lowest loss. > 2. Full 40-meter band coverage but resonant at 7.1 MHz. > 3. Assume using a tuner (like K3 or KAT500). > 4. Choke common mode currents. > > > Total feed line length will be on the order of 100 feet for this particular > dipole. For the 30 meter dipole I also plan to put up, total feed line > length is about 75 feet. My 80 meter dipole will be about 100 feet of > feedline. > > Before I posted this question to this forum, I would have gone ahead and used > the same configuration I used previously when I had my 80 meter doublet up. > That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to > the shack. > > But one motivation for my question is weighing the difference in loss of > tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to > the feed point with a choke balun at that location. > The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 > (at 0.7 dB loss) but I am curious about other solutions. > > 73, phil, K7PEH > > > On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote: > >> Phil, >> >> What do you want to accomplish (other than just an antenna for 40 meters)? >> Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your >> best choice. >> >> Easiest IMHO is to just feed it with coax with a current mode choke at the >> feedpoint. That should present a less than 2:1 SWR over the entire 40 meter >> band. >> >> If you have an extremely long feedline (multiples of a half wave), then you >> might consider using balanced line cut to some multiple of a half wavelength >> (electrical length) and the rest of the way to the shack with coax - put the >> common mode choke at the transition between the balanced line and the coax. >> This latter suggestion takes advantage of the fact that an electrical half >> wave transmission line has the same impedance at both ends - it does not >> matter what the characteristic impedance of the line may be. >> >> But unless you have a very long distance to the antenna feedpoint, the loss >> of practical coax runs is rather low on 40 meters, so the easiest is just to >> use coax. >> >> 73, >> Don W3FPR >> >> On 1/1/2012 12:14 AM, Phil Hystad wrote: >>> OK, lots of discussion on open wire feeders and other ideas so I thought I >>> would throw out a question to soak up some of the Elecraft community >>> knowledge and wisdom. >>> >>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed >>> configuration for single band usage to minimize loss and minimize common >>> mode currents and radiation from the feed line? >>> >>> I am thinking of building such an antenna and I would have no plans for >>> using it on any other band but I would like to use it on the full spectrum >>> of 40 meters with primary focus on the low end for CW. >>> >>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about >>> the best plan to feed it. I am thinking of 450 ohm window line (aka ladder >>> line) to a 1:1 balun and the coax the rest of the way into the shack. Or, >>> how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way >>> into the shack? Would this be a significant difference? Or, some other >>> combination? >>> >>> Or, maybe a 1:1 balun right at the antenna feed point with coax the >>> remaining distance to the shack? >>> >>> Any comments or suggestions? >>> >>> I am thinking of also raising up two other dipoles. I have room to put up >>> 80, 40, and 30 as separate dipole antennas -- it is actually a little bit >>> more awkward to do a fan dipole so I am leaning on not doing that. >>> >>> 73, phil, K7PEH >>> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
